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West Nile Virus Detected in West Tisbury Mosquito

A mosquito found in West Tisbury has tested positive for West Nile virus, the town department of public health announced Thursday afternoon.

The mosquito was caught near Mill Pond, frozen and shipped to the state department of public health for testing earlier this week, and identified as a carrier of the virus by Thursday. The towns of Southampton, Northampton, Barnstable, Melrose and Boston also had West Nile-positive mosquitoes this week.

West Nile is an infectious disease transmitted through mosquitoes, with birds as the most common host. Mosquitoes carrying the West Nile have the potential to spread it to human beings.

Heavy rainfall and consistent periods of heat can contribute to greater prevalence of the virus.

“If you have ignored mosquitoes up until now, once we get into August, it’s really important that now we do something to avoid mosquito bites, and to remind folks that West Nile Virus can occur even in as lovely a place as Martha’s Vineyard,” said state veterinarian Catherine Brown with the Department of Public Health. West Nile is not limited to West Tisbury and can strike in any of the communities on the Island, she said.

West Nile bites pose a serious health risk in a minority of human cases. “The good news is that eight out of 10 people who have been bitten will never realize [the disease],” she said. In most cases, the immune system rejects the virus, and the person never experiences symptoms.

“The remaining two people will have a kind of a flu-like illness,” she said. Those Massachusetts residents who experienced symptoms last summer were sick for two or three weeks, but like most who get West Nile, recovered fully. Only one out of 150 people will experience severe symptoms, where the virus invades the brain, causing more significant illness and, in very rare cases, death. Town departments of public health began circulating information about mosquito-borne diseases and prevention last week.

When spending time outdoors, Ms. Brown recommends that Islanders apply DEET insect repellant or any similar product that is EPA-approved. Residents should also remove any standing water near their home, and replace any broken window screens in their homes.

Ms. Brown said she isn’t surprised that the virus turned up in this region, because once a mosquito tests positive in the state, other contraction events are likely. “It doesn’t tend to be geographically localized,” she said. “Once it arrives in Massachusetts, there is a possibility it could be found anywhere in the Commonwealth. Most cases of human contraction of the virus tend to occur in suburban and urban areas. The infected mosquito was discovered Monday as part of an Islandwide testing program. Each week during summer a pool of 10 mosquitoes are trapped in the northern quadrant of West Tisbury, near Mill Pond, and sent to the state to be analyzed, said West Tisbury health agent John Powers. Additional testing will be performed over the weekend, he said.

The last time West Nile visited the Island was in late July of 2011, when a mosquito collected in Tisbury tested positive for the virus.

Between the years of 2000 and 2008, the towns sent in dead birds for testing, but mosquito trapping replaced that practice. Mosquitoes yield more reliable test results. Testing is performed during the summer months, when mosquitoes are more prevalent.

Horses, llamas and alpacas are also particularly sensitive to the virus, but cats and dogs are generally not at risk.